1. Field of the Disclosure
The present disclosure generally relates to electronic circuits and, more particularly, to an input buffer receiver circuit in a memory device to process reduced-swing inputs.
2. Brief Description of Related Art
Memory devices are electronic devices that are widely used in many electronic products and computers to store data. A memory device is a semiconductor electronic device that includes a number of memory cells, each cell storing one bit of data. The data stored in the memory cells can be read during a read operation. FIG. 1 is a simplified block diagram showing a memory chip or memory device 12. The memory chip 12 may be part of a DIMM (dual in-line memory module) or a PCB (printed circuit board) containing many such memory chips (not shown in FIG. 1). The memory chip 12 may include a plurality of pins or ball contacts 24 located outside of chip 12 for electrically connecting the chip 12 to other system devices. Some of those pins 24 may constitute memory address pins or address bus 17, data (DQ) pins or data bus 18, and control pins or control bus 19. It is evident that each of the reference numerals 17-19 designates more than one pin in the corresponding bus. Further, it is understood that the schematic in FIG. 1 is for illustration only. That is, the pin arrangement or configuration in a typical memory chip may not be in the form shown in FIG. 1.
A processor or memory controller (not shown) may communicate with the chip 12 and perform memory read/write operations. The processor and the memory chip 12 may communicate using address signals on the address lines or address bus 17, data signals on the data lines or data bus 18, and control signals (e.g., a row address select (RAS) signal, a column address select (CAS) signal, a chip select (CS) signal, etc. (not shown)) on the control lines or control bus 19. The “width” (i.e., number of pins) of address, data and control buses may differ from one memory configuration to another.
Those of ordinary skill in the art will readily recognize that memory chip 12 of FIG. 1 is simplified to illustrate one embodiment of a memory chip and is not intended to be a detailed illustration of all of the features of a typical memory chip. Numerous peripheral devices or circuits may be typically provided along with the memory chip 12 for writing data to and reading data from the memory cells 26. However, these peripheral devices or circuits are not shown in FIG. 1 for the sake of clarity.
The memory chip 12 may include a plurality of memory cells 26 generally arranged in an array of rows and columns. A row decode circuit 28 and a column decode circuit 30 may select the rows and columns, respectively, in the array in response to decoding an address provided on the address bus 17. Data to/from the memory cells 26 are then transferred over the data bus 18 via sense amplifiers and a data output path (not shown). A memory controller (not shown) may provide relevant control signals (not shown) on the control bus 19 to control data communication to and from the memory chip 12 via an I/O (input/output) circuit 32. The I/O circuit 32 may include a number of data output buffers or output drivers to receive the data bits from the memory cells 26 and provide those data bits or data signals to the corresponding data lines in the data bus 18. The I/O circuit 32 may also include various memory input buffers and control circuits that interact with the row and column decoders 28, 30, respectively, to select the memory cells for data read/write operations. A prior art memory input buffer receiver circuit is illustrated in FIG. 2 and discussed later hereinbelow.
The memory controller (not shown) may determine the modes of operation of memory chip 12. Some examples of the input signals or control signals (not shown in FIG. 1) on the control bus 19 include an External Clock (CLK) signal, a Chip Select (CS) signal, a Row Address Strobe (RAS) signal, a Column Address Strobe (CAS) signal, a Write Enable (WE) signal, etc. The memory chip 12 communicates to other devices connected thereto via the pins 24 on the chip 12. These pins, as mentioned before, may be connected to appropriate address, data and control lines to carry out data transfer (i.e., data transmission and reception) operations.
FIG. 2 shows a prior art memory input buffer receiver circuit 34 that may be part of the I/O unit 32 in the memory chip 12 in FIG. 1. It is known to one skilled in the art that although one such circuit 34 is shown in FIG. 2, the I/O unit 32 may include additional such receiver circuits as well. The receiver circuit 34 in FIG. 2 includes a single stage (1-stage) differential amplifier pair. The differential pair includes a P-type differential amplifier (P-diff) 36 and an N-type differential amplifier (N-diff) 38. The P-diff 36 includes two PMOS (P-channel Metal Oxide Semiconductor) transistors 40, 42 as input devices to receive the two inputs—a reference signal (Vref) 41 and an input signal (Vin) 43—as shown in FIG. 2. Whereas, the N-diff 38 includes two NMOS (N-channel MOS) transistors 44, 46 as input devices to receive the same two signals—i.e., Vref 41 and Vin 43 signals—as inputs to the differential amplifier 38. The P-type differential amplifier 36 also includes two additional PMOS transistors 47-48 and two NMOS transistors 49-50 for biasing and signal amplification. The connection of these transistors 47-50 with the input devices 40, 42 is shown in FIG. 2 and, hence, not discussed in detail herein for the sake of brevity. Similarly, the additional transistors in the N-diff amplifier 38 include two PMOS devices 51-52 and two additional NMOS devices 53-54, which are connected to the input devices 44, 46 in the manner shown in FIG. 2. Various biasing voltages (Vcc and ground) are also shown for the circuit components in the receiver circuit 34 of FIG. 2.
It is observed here that one of the input signals to the receiver circuit 34 (i.e., the Vin signal 43) may be any signal (data, address, or control) received from a device (e.g., a memory controller (not shown)) connected to the memory chip 12. The input signals may be received by one or more input buffers (not shown) that may be part of the I/O unit 32 in the memory chip 12. The input buffers, in turn, receive the input signals (Vin) and process them through the input buffer receiver circuits similar to the circuit 34 shown in FIG. 2. The reference signal, Vref 41, may also be applied as an input to the differential amplifiers in the receiver circuit 34 to supply a reference voltage signal against which the deviations in the input signal Vin 43 may be compared. For example, the input signal Vin 43 may be considered as having a “large” input swing when, for example, Vin=Vref±150 mV. That is, a signal swing of 300 mV P-P (peak-to-peak) (in the Vin signal) from the reference voltage Vref may be considered a “large” voltage swing. On the other hand, a swing of 75 mV (e.g., Vin=Vref±75 mV) may be considered a “small” voltage swing. As discussed later hereinbelow, the prior art input buffer receiver circuit 34 does not perform well with reduced swing (e.g., Vref±75 mV) input voltages as compared to input voltages with large or medium swings (e.g., Vref±150 mV).
An output of the differential amplifier 36 is obtained at the junction 56A of the drain terminals of the transistors 42 and 50 as shown in FIG. 2. Similarly, an output of the differential amplifier 38 is obtained at the junction 56B of the drain terminals of the transistors 46 and 52 as shown in FIG. 2. Both of these outputs are combined to form the final output of the differential pair 36, 38. This final output is conveniently referred to herein as the Diff_Out signal 56, which is supplied as an input to an inverting configuration (or inverter) 58 before obtaining an output signal 66 therefrom. The inverter 58 includes a pair of PMOS transistors 59-60 and a pair of NMOS transistors 61-62 connected in the manner shown in FIG. 2 to perform a signal inverting function. The Diff_Out signal 56 may be first inverted by the inverting configuration 58 and then by a logic gate inverter 64 before an output 66 is generated from the input receiver circuit 34. Thus, the output signal 66 may be called a “buffered” version of the input signal Vin 43. Such buffering of the input signal Vin 43 received at an input pin 24 of the memory chip 12 may be necessary, as is known in the art, when the input signal is received over a system bus (e.g., the data bus 18) and it may be desirable to “boost” the signal level of the input signal Vin or to generate a “cleaner” version of the input signal Vin before it is supplied to other components (e.g., a row decode circuit 28, or the column decode circuit 30, etc.) of the memory chip 12 for subsequent processing.
It is observed here that when the input signal Vin 43 has a large (e.g., Vref±300 mV) or medium (e.g., Vref±150 mV) swing, the P-diff 36 and N-diff 38 differential amplifier pair in the input stage 34 will generate an output 66 that having symmetrical rising and falling time delays. However, that may not be the case when Vin is of a reduced-swing (e.g., Vref±75 mV) input. When the input signal Vin 43 has a small swing, the output (Diff_Out 56) of the differential amplifier pair 36, 38 will also have less signal swing due to the limited gain of the differential pair 36, 38. Such limited gain may be present because of a large bias current used to increase the operation speed of the differential pair 36, 38, especially when higher clock frequencies are used in the system.
FIG. 3 shows an exemplary set of waveforms in the 1-stage input receiver of FIG. 2 when the input voltage (Vin 43) has a small swing and when there are PVT (process, voltage, temperature) changes. Two waveforms for the Diff_Out signal 56 are shown in FIG. 3—one waveform 56-1 for the situation when there are no PVT changes, and the other waveform 56-2 represents the situation when there is a PVT change. The other two waveforms in FIG. 3 represent the Out signal 66 at the output of the 1-stage input receiver 34. The waveform numbered 66-1 represents the Out signal 66 that corresponds to the Diff_Out signal 56-1 and is related to the situation when there are no PVT changes. The other waveform numbered 66-2 represents the Out signal 66 that corresponds to the Diff_Out signal 56-2 and is related to the situation when there is a PVT change. As discussed before with reference to FIG. 2, the output signal 66 in the receiver 34 is generated from the Diff_Out signal 56 of the differential amplifier pair 36, 38. Therefore, signal swings in the input voltage Vin 43 may affect the output signal 66 through the Diff_Out signal 56, which is generated based on the levels of the input signal Vin 43.
When PVT varies, the center voltage as well as signal swing (from a given Vref) of the Diff_Out signal 56 varies as can be seen from a comparison of the waveforms 56-1 and 56-2 in FIG. 3. In case of reduced-swing inputs (Vin), the switch point of the second stage inverter 58 cannot track the changes in the Diff_Out signal 56 (here, the waveform 56-2). Because of this, the rising and falling time delays in the resulting output signal 66 are unsymmetrical as can be seen from a comparison of the rising and falling edges in the waveforms 66-1 and 66-2. As mentioned before, the 1-stage input receiver 34 may work fine with inputs (Vin) having large or medium swings, but it suffers from performance degradation when the signal swing becomes very small (e.g., Vin=Vref±75 mV).
It is observed that small signal swing is becoming a trend for high bandwidth signal interconnects in electronic systems (including, for example, systems connecting high-speed memory devices with other electronic components through such interconnects). The reduced signal swing can effectively reduce the power supply noise (which may be present in the input signal, Vin), greatly improving the signal integrity. As noted, modern high speed interconnection between a memory chip (e.g., the memory chip 12) and the chipset on a PCB (not shown) also follows the same trend of employing reduced-swing signals to improve signal integrity. However, as per the discussion hereinbefore, it is seen that the prior art input buffer receiver circuit 34 suffers from performance degradation when the input signal swing is very small.
A small or reduced swing input signal (Vin) with high bandwidth thus requires a sensitive and high speed input receiver. It is therefore desirable to devise an input receiver circuit that has improved performance when the input signal swing is small over PVT corners. It is also desirable that such receiver circuit exhibit similar or comparable performance with the prior art receiver circuit of FIG. 2 when the input signal swing is wide. Further, it is preferable that the current dissipation of the new receiver circuit be the same or less than the current dissipation in the previous receiver of FIG. 2.